The next generation of high energy particle accelerator-driven light sources will produce sub-100-femtosecond (fs) high-brightness x-ray pulses. Pump-probe experiments at these facilities require synchronization of pulsed lasers and rf accelerating fields on 100 fs time scales over distances of a few hundred meters to several kilometers. Another example is the International Linear Collider (ILC), which does not have timing requirements as strict but rather must maintain good timing stability over distances greater than 10 km. Transmission of high frequency RF signals over long distances is best accomplished by transmitting RF-modulated light over fiber optic. This avoids interference and signal degradation associated with transmission via coaxial cable or waveguide. In systems where the transmission delay through an RF-over-fiber link must be held constant, delay through the link is typically sensed by retroreflecting part of the signal back to the transmitter, where a mechanical delay is controlled to adjust the optical path length. Such systems suffer from limited mechanical delay range, poor reliability of precision mechanical components, and slow response of the controlled delay. Also, expansion of these RF distribution systems to many channels is difficult due to the complexity of the transmitter, which must incorporate many timing sensors and delays. Some of the currently demonstrated fiber optic timing transmission systems transmit a series of short pulses through the fiber to transmit RF frequencies. The frequencies that can be transmitted this way are limited to harmonics and sub-harmonics of the pulse repetition rate, restricting the frequencies available at the receiver.
There is a need, therefore, to develop improved timing distribution transmission line systems by which solve these problems. Mechanical delays can be eliminated by controlling RF phase electronically, resulting in better reliability and increased delay range, limited only by digital electronics and software. Doing delay sensing and control in the receiver can make high channel count timing systems possible by distributing the delay control function. Use of modulated CW light signals enables arbitrary choice of transmitted frequency and modulation format. Stabilized fiber optic transmission lines allow the possibility of femtosecond level synchronization between various accelerator and laser systems by taking advantage of advances in optical communication technology and metrology.